a) Field of the Invention
The present invention relates to a method for separating at least one amino acid from an aqueous solution of mixed amino acids, which solution is produced, for example, in an amino acid production process such as a synthetic process, a fermentation process, an enzymatic process or a proteolytic process.
b) Description of the Related Art
As adsorption-dependent separation methods for amino acids, ion exchange methods have been known for many years. According to one example of these ion exchange methods, treatment of an aqueous solution of various amino acids with a strongly acidic ion exchange resin is effected by converting the resin into the free form, namely, the H form, causing the resin to adsorb the amino acids thereon and then eluting the amino acids with aqueous ammonia. This method makes use of the fact that an amino acid is an ampholyte, whereby the amino acids are once bonded to functional groups of the ion exchange resin. Accordingly, the pH of the solution of the amino acids is lowered to convert the amino acids into cations, followed by the treatment with the ion exchange resin. The amino acids adsorbed on the resin through ion bonding are then treated with aqueous ammonia as an eluent to raise the pH, so that the cations of the amino acids are subjected to ion exchange with ammonium ions and are hence eluted. Further, the ion exchange resin can be regenerated into the H form with a mineral acid such as hydrochloric acid or sulfuric acid and can then be employed again for the adsorption of amino acids
For example, Japanese Patent Application Laid-Open No. 73050/1981 discloses a method in which one or more aromatic amino acids in an aqueous solution, an aqueous alcohol solution or an alcohol solution are purified using a strongly acidic ion exchange resin of the macroporous type. Even in this method, the ion exchange resin is converted into the H form and, after ionic adsorption of the amino acids, the amino acids are eluted with an aqueous ammonia solution.
Serious drawbacks of such ion exchange methods reside in the use of an acid as a pH-adjusting agent or a regenerating agent and also in the use of aqueous ammonia as an eluent. To practice the above ion exchange methods on an industrial scale, many accompanying problems therefore arise For example, the acid employed must eventually be neutralized and discharged as an effluent from the system. On the other hand, ammonia used as an eluent accompanies amino acids as target substances for the separation Removal of ammonia is therefore indispensable to isolate the amino acids. Further, to discharge the thus-removed ammonia out of the system, it is necessary not only to neutralize the same but also to eliminate by a certain method its nitrogen fraction which causes trouble from the standpoint of environmental protection.